Long-Term (22 Years) Grazing Intensity Research in the Missouri Coteau of North Dakota

Stocking Rate

There are two ways we can express stocking rate, either on a land area or a forage basis. If we express it on a land area basis we are stating how many animals we are putting on a given amount of land for a given length of time. If we express it on a forage basis we are stating how many animals we are allowing to graze a given amount of forage over a given length of time. The drawback of the land area basis is that forage production varies from year to year and place to place, so if we get a year with half the normal forage production we will need to cut the normal stocking rate in half, either by cutting animal numbers in half, cutting the time they graze in half or doubling the amount of land area on which they graze. If we express stocking rate on a forage basis we are keeping the ratio of forage demand to forage supply constant. If we get a year with half of normal forage production, we would still have to cut animals numbers in half, cut the length of time they graze in half or double the amount of land area on which they graze, but the stocking rate would remain the same because we are keeping the ratio of animals to available forage the same.

The unit we use for animal demand is the animal unit month (AUM). An AUM is defined as the forage required for sustaining a 1,000-lb. cow and her calf for one month and assumes they require 26 lbs. of forage a day on a dry matter basis. The animal unit is based on the metabolic weight of the animal, so a 1200-lb. cow would be 1.147 animal units and a 700-lb. steer or open heifer would be 0.765 animal units. For a stocking rate of 1 AUM/acre then we are allowing the equivalent of one mature cow and calf to graze on an acre for one month and for a stocking rate of 3 AUM/acre we are holding the equivalent of three mature cows with calves on one acre for one month, but this is saying nothing about how much forage they will have to graze. For a stocking rate of 1 AUM/ton of forage we are allowing the equivalent of one mature cow and calf to graze on one ton of available forage for one month or 66.6 lbs. per day and for a stocking rate of 3 AUM/ton of forage we are holding the equivalent of three mature cows with calves on one ton of available forage for one month or 22.2 lbs. per day. Table 3 gives an example of stocking rates in AUM/ton of available forage and their equivalent in AUM/acre assuming that the area produces 2,672 lbs./acre, the average of the silty range site in our study.

Table 3. An example of stocking rates in AUM/ton of available forage and the acres of land required to provide that much forage for one month assuming an average year’s forage production on a silty range site (2,672 lbs./acre in an average year). Stocking rate in AUM/acre is the inverse of the number of acres provided.